Electrode grafting by oxidation of an amine catalyzed by a ferrocenyl “antenna” through intramolecular electron transfer

Article abstract of DOI:10.1016/j.elecom.2017.07.021

Two aminoferrocene complexes were studied by electrochemical techniques. Molecules retain the redox properties of both ferrocene and amine groups, but fundamentally different behaviours were observed depending on whether the linker between the two redox end groups was saturated (ethyl bridge) or not (ethynyl bridge). The possibility of an intramolecular electron transfer from the amine to the ferricenium moiety through the π-conjugated linker was demonstrated and the ethynyl bridge is expected to have a dual effect by facilitating both the oxidation of the amine into the cation radical and the production of aminyl radical, due to its strong electron withdrawing effect. Because of this synergy of properties, grafting of the conjugated aminoferrocene complex can occur just by oxidizing the ferrocene group without the presence of a base in solution.

Full text of DOI:10.1016/j.elecom.2017.07.021

Accepted Manuscript
Electrode grafting by oxidation of an amine catalyzed by a
ferrocenyl “antenna” through intramolecular electron transfer
Ewen Touzé, Sylvie Dabos-Seignon, Thomas Cauchy, Frédéric
Gohier, Charles Cougnon
PII:
S1388-2481(17)30204-7
doi: 10.1016/j.elecom.2017.07.021
ELECOM 5996
DOI:
Reference:
To appear in:
Electrochemistry Communications
Received date:
Revised date:
Accepted date:
16 June 2017
12 July 2017
26 July 2017
Please cite this article as: Ewen Touzé, Sylvie Dabos-Seignon, Thomas Cauchy, Frédéric
Gohier, Charles Cougnon , Electrode grafting by oxidation of an amine catalyzed by
a ferrocenyl “antenna” through intramolecular electron transfer, Electrochemistry
Communications (2016), doi: 10.1016/j.elecom.2017.07.021
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ACCEPTED MANUSCRIPT
Electrode grafting by oxidation of an amine catalyzed by
a ferrocenyl “antenna” through intramolecular electron
transfer
Two aminoferrocene complexes were studied by electrochemical techniques. Molecules retain the redox
properties of both ferrocene and amine groups, but fundamentally different behaviours were observed
depending on whether the linker between the two redox end groups was saturated (ethyl bridge) or not
(ethynyl bridge). The possibility of an intramolecular electron transfer from the amine to the ferricenium
moiety through the -conjugated linker was demonstrated and the ethynyl bridge is expected to have a
dual effect by facilitating both the oxidation of the amine into the cation radical and the production of
aminyl radical, due to its strong electron withdrawing effect. Because of this synergy of properties,
grafting of the conjugated aminoferrocene complex can occur just by oxidizing the ferrocene group
without the presence of a base in solution.

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Amine is a key surface attachment functionality to prepare modified surfaces for sensor applications due
to its presence in a wide variety of biomolecules. Particularly, primary amines give a radical cation upon
oxidation and after deprotonation, favoured by the presence of a base in solution, the radical aminyl
obtained can react with carbon and metallic surfaces to be covalently attached [1]. Unfortunately, due to
the poor delocalization of the charge in the amino cation-radical, the oxidation of aliphatic amines
occurs most often beyond 1 V/SCE, that is hardly accessible in physiological media [2]. Because of a
better delocalization of the charge, the oxidation of aromatic primary amines becomes easier and
grafting is possible in aqueous media [3]. Recently, a conjugated amino-ferrocifen complex could even
be grafted at very low potential, by oxidizing the ferrocene (0.4 V/SCE) through an intramolecular
electron transfer from the amine to the ferricenium moiety acting as a redox shuttle [4,5]. Here, we
studied the electrochemical behaviour of two aminoferrocene complexes, having saturated and
unsaturated bridges, in order to demonstrate the impact of the conjugation on the grafting efficiency.
Computational modelling and electrochemical techniques were applied to gain insight into the behaviour
of molecules following the one-electron oxidation of the ferrocene moiety, while electrodes modified at
different potential values were investigated by

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electrodes were sonicated for 10 min in acetonitrile before to be studied
XPS measurements were performed with a Kratos Axis Ultra spectrometer using a Al K
monochromatic beam working at 1486.6 eV. All spectra were recorded in the constant energy mode at a
pass energy of 20 eV. Data treatment was performed with CasaXPS software and all spectra were
calibrated taking 284.5 eV (graphite-like carbon) as a reference binding energy.
AFM studies were carried out using the NanoObserver microscope from CSInstruments in tapping
mode. The acquired images were processed with the Gwyddion free SPM data analysis software.
For all calculations the Gaussian 09 software (revision A.02) was used.
after the charge reorganization following the first one-electron
oxidation
.
Atomic charge mapping was obtained by Hirshfeld population analysis of density functional electronic
charge distributions calculated from the 6-311+G(2df,2pd) basis set.
Recently, multicomponent molecular assemblies incorporating the ferrocenyl group as an electron donor
were extensively used to explore the intramolecular electronic interactions in donor-acceptor conjugated
systems suitable for nonlinear optical (NLO) materials, metal ion recognition applications and electrode
surface modifiers [6]. Regarding this latter issue, we have studied the electrografting efficiency of two
ferrocenyl-aniline derivatives having a saturated or unsaturated spacer. Figure 1 presents the cyclic
voltammograms (CVs) of molecules in acetonitrile solution and the computational modelling of the two
aminoferrocene complexes upon the first one-electron oxidation.

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1
2
(
a)
b)
3
(
10 µA
1
2
(
c)
3
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
E (V) vs. Fc+
/0
The two aminoferrocene complexes retain the redox characteristics of both ferrocene and amine groups,
with features of reversible and irreversible one-electron transfers. When the ferrocene and the amine
groups are connected through a saturated bridge, the formal potential of the ferrocene, E°Fc, is almost
independent of the aminophenyl ring (E°Fc = - 0.019 V). However, this potential is centered at E°Fc
=
0
.112 V for the
compared to the phenylethynylferrocene (E°Fc = 0.184 V), demonstrating an intramolecular
interaction between the two redox-end groups. In addition, when consecutive CVs were recorded in
the solution, a rapid decrease of the oxidation wave of
the amine was obtained, in good agreement with the electrode passivation by attack of aminyl radicals.

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By analogy, consecutive CVs obtained with the
derivative
show a better stability, and the presence of 2,4,6-trimethylpyridine (collidine) as a base was needed to
passivate the electrode surface by facilitating the deprotonation of the electrogenerated amino cation-
radical into the aminyl radical (results not showed). These results demonstrate that the ethynyl bridge
have the dual effect of improving the electronic contact between the two redox-end groups and fostering
the production of aminyl radicals by facilitating the deprotonation of the amino cation-radical due to its
strong electron withdrawing nature. In addition, the theoretical modelling of the charge distribution
occurring in the aminoferrocene complexes following the one-electron oxidation of the ferrocenyl
moieties, shows the presence of a fraction of positive charge density on the nitrogen atom when an
ethynyl bridge exist (Figure 1), implying an intramolecular electron transfer from the amine to the
electrogenerated ferricenium moiety. Nonetheless, the intra-molecular electron transfer is assumed to be
very slow, since the oxidation of the ferrocenyl moiety for the -conjugated aminoferrocene complex
remains reversible in the cyclic voltammetry time-scale even at 50 mV/s (CVs in dotted line in Figure
1
).
Under such conditions, the attachment of the aminoferrocene complex could occur via the oxidation of
the ferrocene group. To verify this assumption, a GC electrode was polarized for 5 min at different
potential values in 1 mM aminoferrocene acetonitrile solutions. After thorough rinsing, the GC electrode
was transferred in a free acetonitrile solution to be studied. Figure 2 presents the relation between the
electrode surface coverage, determined by integrating the CV recorded at 100 mV/s in acetonitrile, and
the electrode potential. Typical CVs recorded in acetonitrile for electrodes polarized at 0.027 V, 0.927 V
and at potential values corresponding to maximum surface coverages, are superimposed.

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2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(
a)
(b)
(c)
0
.4 µA
0.4 µA
0.4 µA
-0.3
0.0
0.3
-0.5 -0.3 0.0 0.3 0.5
-0.3
0.0
0.3
-0.4
-0.2
0
0.2
0.4
E (V) vs Fc+
0.6
0.8
1
1.2
/0
Importantly, after electrolysis at potential values where only the ferrocene group was oxidized, GC
electrodes were solely modified when an unsaturated bridge exists. In this low-potential range, the
electrode coverage was found equal to ca. 0.6 x 10-10 mol/cm for the
2
compared to ca. 0.1 x 10-10 mol/cm for the saturated derivative, which can be
2
,
attributed either to an inefficient electron transfer through the saturated bridge or to a spontaneous
nucleophilic addition of the amine on the carbon surface. Note that CVs recorded on modified electrodes
show a peak-to-peak potential separation minus than 20 mV at 100 mV/s, that agrees well with surface-
confined molecules. In addition, the fact that the potential offset for the response of the ferrocenyl
attached complexes is maintained, proves that the ethynyl bridge is preserved during the grafting. These
results are in good agreement with those obtained by Amatore et al. with a conjugated amino-ferrocifen
complex, for which the grafting was possible from the oxidation of the ferrocene [4,5].

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When the amino groups start to be oxidized, the electrode coverage rapidly increases to a maximum,
exceeding a surface coverage of 1.3 x 10-10 mol/cm , before decreasing dramatically, falling nearly to
zero beyond 1 V. For the conjugated aminoferrocene complex, the net increase in the electrode coverage
when GC electrode is polarized at the threshold potential of the oxidation of the amine indicates that the
grafting process is more efficient when the amino group is directly oxidized. However, the rapid
decrease in the surface coverage for applied potential values exceeding that of the amine remains
unclear. This sudden drop in redox activity is inconsistent with a progressive electrode passivation
resulting in the growth of a poorly conducting organic layer, and by analogy with a recent work, we
tentatively suggest that the organic layer generated in this high-potential range totally blocks the
electrode in the potential window investigated [7].
2
For a better surface characterization, XPS and AFM experiments provide information on the film
composition and its “dry state” thickness, depending on the potential applied (Figure 3). In order to
obtain unambiguous results, longer electrolysis (one hour) in concentrated solutions (5.9 mM) were
performed. As demonstrated by depth profiling achieved on modified PPF working electrodes by
scratching a small section of attached films with an AFM tip, multilayer films were obtained with the
unsaturated-bridged ferrocene complex by direct or indirect oxidation of the amine (Figures 3B and 3C).
By contrast, when the spacer group is saturated, only the direct oxidation of the amine gives a multilayer
film (Figure 3F). In these experiments, the three lowest depth profiles are assumed to be due to non-
radical electrochemically induced reactions, such as spontaneous nucleophilic addition of the amine. In
the current state of our knowledge, it seems to us to be hazardous to compare and discuss the
thicknesses of grafted films, because a strong heterogeneity was observed in a similar study [5].
However, the main conclusion is that a multilayer film can be obtained by oxidizing the ferrocenyl
moiety of the unsaturated-bridged ferrocene complex, confirming the possibility of an indirect oxidation
of the amine.

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Eapplied = E° - 400 mV
E
= E°
Eapplied = E° + 400 mV
Fc
applied
Fc
Fc
A
B
C
0.71 ± 0.12 nm
13.73 ± 0.6 nm
8.56 ± 0.17 nm
0
1
2
0
1
2
3
0
1
2
x (µm)
x (µm)
x (µm)
5000 CPS
D
E
F
1.74 ± 0.14 nm
40 ± 2 nm
1.78 ± 0.3 nm
0
1
2
0
1
2
3
0
1
2
3
4
x (µm)
x (µm)
x (µm)
716
712
708
704
716
712
708
704
716
712
708
704
Binding energy (eV)
Binding energy (eV)
Binding energy (eV)
XPS spectra of Fe 2p3/2 core levels as a marker of molecules in the grafted films, and especially the
Fe(III)/Fe(II) atomic ratio, is very instructive about the intramolecular redox process (Figure 3). When
GC sheets were assumed to be just spontaneously modified (Figures 3A, 3D and 3E), the XPS spectra
consist mainly in a peak ascribed to Fe(II) (at 707.9 eV) or Fe(III) (at 711 eV), according to the GC sheet
is biased at a potential where the ferrocenyl moiety is in its reduced or oxidized form. In contrast, it is
noteworthy that the ferrocenyl moiety is attached to the surface in mixed valence states when multilayer
films were obtained, even though the GC sheets were biased at potential values where the ferrocenyl
group is oxidized. In such cases, the existence of ferrocene in reduced form may be explained by the
ferricenium reduction via an intramolecular electron transfer from the amine (Figure 3B) or by reaction
of the ferricenium moiety with a radical species, giving a substituted ferrocene [8]. The present results

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do not distinguish between these two mechanisms and further experiments designed to probe this
question are in hand.
.
We thank the French Ministry for the PhD grant of E.
Touzé.

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). Computational
modelling of the charge distribution following the first one-electron oxidation was superimposed for the
two aminoferrocene complexes.
and
average of the peak potentials of the reversible ferrocene/ferricenium couple
. Note that E°Fc is defined as the geometric

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

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


An intramolecular electron transfer from an amine to a ferricenium moiety is demonstrated
A ferrocenyl ‘antenna’ makes it possible to graft an amino compound at low anodic potential
Preparation of surface molecular assemblies of unsaturated bridged ferrocenyl moieties